which I have observed—and many others no doubt will be discovered—throw some light on this subject. When ducks suddenly emerge from a pond covered with duckweed, I have twice seen these little plants adhering to their backs; and it has happened to me, in removing a little duckweed from one aquarium to another, that I have unintentionally stocked the one with freshwater shells from the other. But another agency is perhaps more effectual: I suspended the feet of a duck in an aquarium, where many ova of freshwater shells were hatching; and I found that numbers of the extremely minute and just-hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These just-hatched molluscs, though aquatic in their nature, survived on the duck’s feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and if blown across the sea to an oceanic island, or to any other distant point, would be sure to alight on a pool or rivulet. Sir Charles Lyell informs me that a Dyticus has been caught with an Ancylus (a freshwater shell like a limpet) firmly adhering to it; and a water-beetle of the same family, a Colymbetes, once flew on board the Beagle , when forty-five miles distant from the nearest land: how much farther it might have been blown by a favouring gale no one can tell.
With respect to plants, it has long been known what enormous ranges many freshwater, and even marsh-species, have, both over continents and to the most remote oceanic islands. This is strikingly illustrated, according to Alph. de Candolle, in those large groups of terrestrial plants, which have very few aquatic members; for the latter seem immediately to acquire, as if in consequence, a wide range. I think favourable means of dispersal explain this fact. I have before mentioned that earth occasionally adheres in some quantity to the feet and beaks of birds. Wading birds, which frequent the muddy edges of ponds, if suddenly flushed, would be the most likely to have muddy feet. Birds of